Modular two-level voltage source converter for direct current transmission systems

This paper presents a modular two-level voltage source converter (M2L-VSC) suitable for short distance dc transmission systems with relatively low dc operating voltage. The proposed M2L-VSC consists of two sets of three-phase cells, with each cell has its own capacitor, and these capacitors do not discharge when converter is blocked during dc short circuit fault. The main attributes of the proposed M2L-VSC are absence of the 2nd order harmonic from the arm currents, and reduced energy storage requirement as the cell capacitor only experience high-order harmonic currents associated with the switching frequency as in conventional two-level converter (C2L). The technical feasibility of the M2L-VSC for dc transmission systems has been assessed using simulations and corroborated experimentally. It has been shown that the transient responses of M2L-VSC to ac and dc faults are similar to that of the modular multilevel converter (MMC)